FIG. 1 is a cross-sectional view illustrating the fundamental construction of a wet-type multiplate clutch 10, which serves to transmit power between a clutch casing 21 and a hub 22. FIG. 1 shows a spline groove 23 maintained in fitting engagement with separator plates 30, a spline groove 24 maintained in fitting engagement with friction plates 40, a piston 25 for pressing the separator plates 30 and friction plates 40, a backing plate 26 against which the plates 30,40, a snap ring 27 supporting the backing plate 26, and a sealing ring 28 for the piston 25.
In the wet-type multiplate clutch 10, the friction plates 40 and separator plates 30 are alternately arranged between the clutch casing 21 and the hub 22. Engagement or disengagement of the clutch 10 is effected by pressing the friction plates 40 and separator plates 30 with the piston 25 or by canceling the pressing of these plates 40,30.
Recently, there is an ever-increasing demand for improvements in the fuel economy of automobiles. Keeping in step with this trend, there is an outstanding demand for a further reduction in the idling drag between friction plates and separator plates during non-engagement of a clutch in an automatic transmission.
There have hence been contemplated friction plates provided with friction linings, each of which has one or more oil grooves having closed ends to separate the friction plates from their associated separator plates during non-engagement of a clutch and also has one or more oil passages extending radially through the friction lining to feed lube oil onto a friction surface for the prevention of seizure during engagement of the clutch. (see, for example, JP-A-11-141570 and JP-A-2005-76759)
To improve the shift response in an attempt to make not only an improvement in fuel economy and but also improvements in engine performance, the clearances between friction plates and their associated separator plates have become smaller in recent years than before, tending to result in a greater drag torque due to intervening oil films during idling.
With such conventional friction plates, no sufficient drainage of lube oil is feasible on their friction surfaces, thereby failing to satisfactorily meet the demand for a further reduction in drag torque. Especially during rotation in a low-speed range, the drainage of intervening lube oil between the friction plates and the associated separator plates is not sufficient so that the drag torque cannot be reduced.